1. Field of the Invention
This invention relates generally to magnetoresistive (MR) read head sensing elements for magnetic data recording devices and, more particularly, to a rapid MR read head width measurement technique suitable for use during manufacture of data recording head assemblies.
2. Description of the Related Art
Computer system secondary data storage is commonly provided in the form of a direct access storage device (DASD), such as a hard disk drive, a tape drive subsystem, or the like. A typical hard disk drive unit includes one or more rotating storage disks on which digital data is stored magnetically in a plurality of concentric tracks. Small read/write heads are positioned close to the rotating disk surface and moved from track to track to transfer data between the computer system and the spinning storage disk. Similarly, a typical tape drive unit includes a flexible magnetic tape on which data is stored magnetically in a plurality of parallel tracks. The tape is streamed over a small interleaved read/write head array to transfer data between the computer system and the tracks of the streaming tape. DASD read/write heads are usually manufactured by depositing various thin-films on a substrate to form an array of read head magneto-resistive (MR) sensor elements interleaved with write head magnetic gap elements. This substrate is then sliced and the pieces polished and mounted to produce read/write heads having the desired number of interleaved read and write elements for use in hard disk drives, tape drives, or the like.
An MR head includes a center portion denominated the MR stripe, which is the element that senses changes in magnetic field representing data stored on a magnetic disk or tape surface in a DASD. MR read head sensors are well-known in the art and are particularly useful as read elements for high data recording densities. The MR read sensor provides a higher output signal than an inductive read head, which results in a higher signal-to-noise ratio for the playback channel, thereby permitting the reading of a higher areal density of recorded data on the magnetic disk surface. Such high data recording densities are possible because the MR sensor typically is very small (a 1 xcexcm long stripe face with a read-width of 10 nm or less is typical). Because of the small sizes involved, modern MR read sensor fabrication is accomplished using monolithic thin film photolithographic fabrication technology. FIG. 5 illustrates a typical thin film wafer substrate on which an interleaved array of MR read sensor elements and magnetic gap elements have been fabricated. In the present art, 16,000 or more such interleaved read/write head pairs are fabricated on a single wafer substrate during one fabrication procedure.
The sensitivity of an MR head depends on many factors. One of the most significant factors is the bias current provided to the MR head. The ability to read a signal from the storage media is, in part, a function of the amount of bias current supplied to the MR head. Signal sensitivity can be increased by increasing the amount of bias current supplied to the MR head. Therefore, increased bias current generally produces an improved signal-to-noise ratio and reduces bit error rates. However, simply increasing the bias current is not a complete solution because excessive current can significantly shorten the useful life span of the MR read head.
Bias current can adversely affect MR read head life in two different ways. First, application of bias current in excessive quantities can cause the MR sensor element to overheat. If the current density reaches a high enough level, the MR element can burn out. This type of catastrophic failure is typically avoided by selecting a MR element bias current that avoids burnout over the entire operating temperature range.
Catastrophic failure, however, is not the most common cause of MR read head failure. The most common cause of MR read head failure is a phenomenon known as electromigration and/or interlayer diffusion. Constant exposure to even normal operating levels of bias current will, over time, change the molecular structure of the MR sensor element, thereby degrading the magnetic sensing capability of the MR read head.
During manufacture, the MR read heads are typically characterized to determine the range of their operating characteristics over temperature and bias current variations. The performance of MR heads fabricated on a given fabrication line may vary considerably because of process variations in the important geometric features on the heads, such as read-width (RW) and stripe height (SH). Proper Quality Control (QC) procedures must assure that even the MR read head with the worst-case geometric tolerances can provide a minimum desired lifetime. One option is for the designer to select a MR stripe bias current that holds the stripe temperature below a predetermined threshold, thereby providing the desired minimum lifetime for the DASD unit but at the expense of reducing MR sensor performance by producing all production heads for operation with the bias current selected for the worst-case element geometry.
MR heads with sensor elements that fall within a nominal range of manufacturing tolerances may be driven with higher bias currents to boost their performance without exceeding the relevant temperature thresholds, but such higher bias current can shorten the life expectancy of the MR heads at the edge of expected manufacturing variations. Because an MR head should be operated below 155 degrees Celsius to avoid premature failure, the maximum bias current for all heads is typically set to equal the maximum bias current for the worst-case head.
Because of the tradeoff between performance, bias current and element life expectancy, MR sensor element performance depends critically on the geometry of the MR stripe. Two important MR stripe dimensions are the read-width (RW) and the stripe height (SH), as they are known in the art. The MR read-width (RW) has an important effect on DASD performance because it directly affects the minimum available track width and, therefore, the maximum areal storage density for the DASD. Normally, in the art, the RW for a MR sensor can be measured only during production by using electron microscopy to visually inspect the physical MR element. Such a technique is unacceptably slow and burdensome for quality-control (QC) inspection in a MR element production line capable of fabricating 16,000 or more elements per wafer substrate. As a result of this problem, the usual production QC practice is to select a few exemplary MR elements from each wafer substrate for inspection by electron microscopy. This means that 99% or more of the MR read head elements are not properly inspected for acceptable RW value, requiring the bias current design compromises discussed above to accommodate expected geometric variations.
There is accordingly a clearly-felt need in the art for a reasonably efficient technique for testing all MR sensor elements for proper stripe geometry during production. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.
This invention solves the problem described above by providing for the first time a method for the automated production screening of the read-width (RW) and/or the stripe-height (SH) for every magnetoresistive (MR) read sensor element in a wafer substrate. The method of this invention uses the RW and/or SH values (optical SH measurements require the cross-sectioning of the sensor for measuring) found by optical examination with electron microscopy of several of the MR sensor elements to estimate two substrate coefficients that relates the optical RW and SH measurements to heating-delta measurements, xcex4=(RHxe2x88x92RC)/RC, where RH is the RC is the sensor resistance when cold, both of which can be measured using automated equipment These relationships are sufficiently similar among all MR sensor elements manufactured on a single wafer substrate during a single manufacturing procedure that, when the hot resistance RH is measured at a constant applied voltage, the heating-delta, may be used with a first substrate coefficient to estimate the read-width RW of each MR sensor element for quality-control purposes during manufacture. When the hot resistance RH is measured at a constant applied current, the heating-delta may be used with a second substrate coefficient to estimate the stripe-height SH of each MR sensor element for quality-control purposes during manufacture.
It is a purpose of this invention to use such a substrate coefficient to estimate the read-width RW from a first heating-delta measured for any MR read sensor element on the same substrate. When the hot resistance RH is measured at a constant applied voltage, the resulting heating-delta, which can be measured using automated equipment, may be used with a first substrate coefficient to estimate the read-width RW of each MR sensor element for quality-control purposes during manufacture.
It is an advantage of this invention that the read-width RW for every MR sensor element can be estimated from the optical measurement of RW for a few of the MR sensor elements and the automated measurement of a cold resistance and a constant-voltage hot resistance for all MR sensor elements on the substrate wafer.
It is another purpose of this invention to use another substrate coefficient to estimate the throat height SH from a second heating-delta measured for any MR read sensor element on the same substrate. When the hot resistance RH is measured at a constant applied current, the resulting heating-delta, which can be measured using automated equipment, may be used to estimate the stripe-height SH of each MR sensor element for quality-control purposes during manufacture.
It is another advantage of this invention that the throat height SH for every MR sensor elements can be estimated from the optical measurement of SH for a few of the MR sensor elements and the automated measurement of a cold resistance and a constant-current hot resistance for all MR sensor elements on the substrate wafer.
In one embodiment, this invention is a method for determining the read-width RWi of one of a first plurality (I) of magnetoresistive (MR) sensor elements fabricated on a substrate, including the steps of optically measuring the read-width RWk of each of a second plurality (K less than I) of the MR sensor elements, where ixe2x89xa0k=1, K, measuring a heating-delta xcex4i for the one MR sensor element and a heating-delta xcex4k for each of the second plurality (K less than I) of MR sensor elements and calculating the read-width RWi of the one MR sensor element as a function of the optical read-width measurements RWk and the heating-delta measurements xcex4i and xcex4k.
In another embodiment, this invention is a method for determining the stripe-height SHi of one of a first plurality (I) of magnetoresistive (MR) sensor elements fabricated on a substrate, including the steps of optically measuring the stripe-height SHk of each of a second plurality (K less than I) of the MR sensor elements, where ixe2x89xa0k=1, K, measuring a heating-delta xcex4i for the one MR sensor element and a heating-delta xcex4k for each of the second plurality (K less than I) of MR sensor elements, and calculating the stripe-height SHi of the one MR sensor element as a function of the optical stripe-height measurements SHk and the heating-delta measurements xcex4i and xcex4k.
In yet another embodiment, this invention is a process for manufacturing magnetoresistive (MR) heads having a read-width RWj within a predetermined range (RWmax, RWmin) including the steps of fabricating a first plurality (I) of magnetoresistive (MR) sensor elements on a substrate, optically measuring the read-width RWk of each of a second plurality (K less than I) of the MR sensor elements, where k=1, K, measuring a heating-delta xcex4i for each of the first plurality (I) of MR sensor elements, calculating an upper heating-delta limit xcex4max and a lower heating-delta limit xcex4min for the MR sensor elements as a function of the optical read-width measurements RWk and the heating-delta measurements xcex4k, and selecting those MR sensor elements for which the heating-delta xcex4j is within the range (xcex4max, xcex4min) for use in making the magnetoresistive (MR) heads.
In still another embodiment, this invention is a process for manufacturing magnetoresistive (MR) heads having a stripe-height SHj within a predetermined range (SHmax, SHmin) including the steps of fabricating a first plurality (I) of magnetoresistive (MR) sensor elements on a substrate, optically measuring the stripe-height SHk of each of a second plurality (K less than I) of the MR sensor elements, where k=1, K, measuring a heating-delta xcex4i for each of the first plurality (I) of MR sensor elements, calculating an upper heating-delta limit xcex4max and a lower heating-delta limit xcex4min for the MR sensor elements as a function of the optical stripe-height measurements SHk and the heating-delta measurements xcex4k, and selecting those MR sensor elements for which the heating-delta xcex4j is within the range (xcex4max, xcex4min) for use in making the magnetoresistive (MR) heads.
In another embodiment, this invention is a process for manufacturing read/write heads for direct access storage devices (DASDs) including magnetic gap write elements interleaved with magnetoresistive (MR) sensor read elements each having a read-width RWj within a first predetermined range (RWmax, RWmin) and a stripe-height SHj within a second predetermined range (SHmax, SHmin) including the steps of fabricating a first plurality (I) of MR sensor elements on a substrate optically measuring the read-width RWk of each of a second plurality (K less than I) of the MR sensor elements, where k=1, K, measuring a first heating-delta xcex4i for each of the first plurality (I) of MR sensor elements, calculating a first upper heating-delta limit xcex4max and a first lower heating-delta limit xcex4min for the MR sensor elements as a function of the optical read-width measurements RWk and the first heating-delta measurements xcex4k, measuring the stripe-height SHk of each of a second plurality (K less than I) of the MR sensor elements, where k=1, K, measuring a second heating-delta xcex4i for each of the first plurality (I) of MR sensor elements, calculating a second upper heating-delta limit xcex4max and a second lower heating-delta limit xcex4min for the MR elements as a function of the optical stripe-height measurements SHk and the second heating-delta measurements xcex4k, and selecting those MR sensor elements for which the first heating-delta xcex4j is within the first range (xcex4max, xcex4min) and the second heating-delta xcex4j is within the second range (xcex4max, xcex4min) for use in making the read/write heads.
The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing.